In magnetic data storage devices such as disk drives and tape drives, analog signals being read from a magnetic medium (disk or tape) are typically amplified, differentiated and then passed through a zero crossing detector for conversion back into a binary signal. The peaks of the raw signal before amplification become zero crossings after differentiation. The digital information is contained in the zero crossing timing. Therefore, the digital information is contained in the timing of the unamplified peaks and not in the amplitude of the peaks. If the gain of the amplifier is too high, the peaks may become distorted or clipped so that critical timing information is lost. If the gain is too low, some peaks may not be detected so that data is missed.
It is common for some drives to record an initialization area on the magnetic medium for head alignment adjustment and for calibration of the gain of the analog amplifiers. Gain calibration is typically done once when the power is turned on or when a new medium is inserted. For example, for tape drives, the initialization signal is typically a single frequency burst recorded close to the beginning of tape and before the data area of the tape. This initialization area is typically recorded once when the medium is formatted and is not rewritten unless the medium is reformatted.
When a magnetic medium having magnetized areas passes over a magnetic head, the magnetized areas of the medium are partially erased. In addition, even with no usage, some self erasure may occur over time. Over time, with repeated passes, the amplitude of a raw signal from the magnetized areas may eventually degrade by as much as 25%. With a properly adjusted analog read circuitry gain, a 25% degradation in analog signal amplitude will not result in a corresponding degradation of digital data. However, if the analog read circuitry gain is calibrated from a calibration signal amplitude which has degraded such that it is 25% lower than the signal amplitude of more recently written data areas, the analog gain may be set too high. This may distort the data signals sufficiently to degrade digital data integrity.
There is a need for analog gain calibration using actual data signals. This has three potential advantages. First, data fields are less subject to degradation because individual data fields on average do not see as many passes of the head as the area which is used for initial alignment. Second, data fields are less subject to degradation because they are typically rewritten frequently. Finally, regardless of amplitude, the data fields contain the actual information which needs to be extracted.
Use of actual random data signals for amplitude calibration has a problem which must be overcome. The analog signal amplitude varies with frequency. A calibration reference burst is typically a single frequency resulting in a single amplitude. Actual random data fields have varying frequency depending on the digital data pattern. Therefore, when using actual random data, there must be a way to calibrate gain using a varying amplitude calibration signal. Continuous automatic-gain-control (AGC) circuits may be used. However, in digital recording, AGC must typically be disabled for initial head alignment and media verification. In addition, some sort of gain hold must be provided when the head passes over non-data or erased areas of the medium. Providing circuitry for setting gain with AGC disabled and circuitry for holding gain may add complexity. The present invention provides an alternative method which provides an optimized fixed gain.